Abstract
It is well-accepted that supplementary cementitious materials (SCMs) are the most scalable way of reducing cement-driven carbon dioxide emissions of concrete. Because of changing industrialization trends, production of conventional SCMs such as fly ash and slag have been reduced in many locations to a point at which supply often does not meet demand. Therefore, it is critical to identify, characterize, and ultimately specify novel SCMs for their use in concrete. Although SCMs have many effects on concrete properties, arguably their most important effects are improvements in long-term mechanical properties and durability, driven by pozzolanic and latent hydraulic reactions. Thus, it is critical to differentiate between pozzolanically/hydraulically reactive and unreactive materials and to quantify the level of reactivity relative to the spectrum of available SCMs. The issues with ASTM International tests such as strength activity index (SAI) test, which are indirect and sometimes misleading measures of reactivity, are well known. In the last 10 years, many improved reactivity tests have been developed, including the R3 test (recently standardized as ASTM C1897-20, Standard Test Methods for Measuring the Reactivity of Supplementary Cementitious Materials by Isothermal Calorimetry and Bound Water Measurements) and its variants, and tests based on lime strength. This special issue brings together 14 journal papers primarily focused on measuring SCM reactivity and the link between reactivity and property development in cement-SCM pastes, mortars, and concrete. Multiple papers focus on various reactivity tests: the SAI test, the R3 test, the modified R3 test, and resistivity measurements. Although the tests differ, it was generally found that reactivity tests such as the R3 and bulk resistivity tests can more accurately differentiate between reactivity and unreactive materials and may provide some quantification of total reactivity compared with the SAI test. Some authors focused more on material reactivity, providing detailed reactivity studies of one type of material (for example, harvested Class C fly ash), or comparison of the reactivities of multiple materials such as nontraditional SCMs, natural pozzolans, or blended SCMs. Multiple authors focused on durability and performance, and it was found that alkali-silica reaction (ASR) expansion results correlated reasonably well with reactivity and resistivity measurements. In one paper, the available alkali test from ASTM C311/C311M-22, Standard Test Methods for Sampling and Testing Fly Ash or Natural Pozzolans for Use in Portland-Cement Concrete, was studied in detail, and it was suggested that the current test could provide an acceptable indication of system free alkalis to enable wider use. Finally, two papers utilized machine learning approaches to predict SAI or surface resistivity based on characteristic properties of the fly ash. Although not comprehensive, a few additional interesting findings from the published works are highlighted here:
SAI and the accelerated mortar bar test for ASR, used together, were found to provide differentiation between inert and reactive materials and were proposed as a screening tool.
Many materials that met ASTM C618, Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete, did not provide mitigation of ASR damage, perhaps resulting from poor reactivity related to high iron oxide contents.
Regardless of test method used, calcined clays (with moderate-high kaolinite content) are significantly more reactive than most other SCMs.
Limits on loss on ignition of harvested Class C fly ash may aid in improving their performance when used in concrete.
The degree of reaction of SCMs in cementitious systems can be accurately quantified using quantitative X-ray powder diffraction approach. Although this special issue provides an excellent overview of the state of the art of use of reactivity quantification methods and performance of reactive and nonreactive materials, papers included herein are primarily applied in nature and do not necessarily aim to identify fundamental mechanisms driving SCM reactivity. Dissolution/reactivity of model calcium aluminosilicate glasses, molecular dynamics and other related modeling approaches, and reactivity in the presence of chemical admixtures could all be topics for a follow-up special issue.
